An external trip indicator for an oil-immersed circuit breaker requires transmission of the open/closed state of the breaker contacts through the wall of the oil-filled housing that encloses the breaker contacts.
Mechanical trip indicators are generally preferred for use in circuit breakers because a simple mechanical coupling provides a more direct and reliable indication of the state of the breaker contacts. However, prior art mechanical trip indicators with external display used in oil-immersed circuit breakers require an oil-seal around the mechanical component that passes through the wall of the oil-filled housing, and such oil-seals are prone to leak.
Electrical trip indicators avoid the problems associated with leak-prone oil-seals. However, prior art electrical trip indicators are subject to the risk of electrical component failure and power source failure.